Monitoring Kittlitz’s and marbled murrelets in Glacier Bay National Park and Preserve: 2019 annual report

Hoekman, Steven; Johnson, William

Since 2009, the National Park Service’s Southeast Alaska Network (SEAN) has monitored abundance and spatial distribution of Kittlitz's (Brachyramphus brevirostris, “KIMU”) and marbled murrelets (B. marmoratus, “MAMU”) in Glacier Bay National Park and Preserve, an important summer residence for both species. The design of the monitoring program focuses on KIMU, with secondary consideration of MAMU. The SEAN uses vessel-based line transect surveys to estimate species-specific, on-water density and abundance of murrelets, accounting for detection probability and partial identification (identification only to genus Brachyramphus). We surveyed 40 transects totaling 206.6 km from 6-13 July 2019 across the 1,170 km2 survey area in Glacier Bay proper. We estimated an abundance of 4,155 KIMU (SE = 934) and 64,868 MAMU (SE = 12,725). Estimated MAMU abundance was 24% lower than 2018 and near the 11-year monitoring average. In contrast, estimated KIMU abundance was 60% lower than 2018 and, at <40% of the 11-year average, the lowest during SEAN monitoring. The spatial distribution of KIMU was also atypical. Few to no KIMU were observed in upper fjords of the East and West Arms, including historically consistently-used areas in Reid Inlet, around Russell Island, and in the Hugh Miller-Scidmore Area. Instead, KIMU aggregated almost exclusively near the mouth of Muir Inlet and on the east side of the Central Bay. Consistent with prior results, MAMU congregated in the Central and Lower Bay. However, MAMU mirrored KIMU in decreased use (relative to prior surveys) of the East and West Arms and of the Hugh Miller-Scidmore Area and in increased use of the upper east side of the Central Bay, suggesting common ecological drivers to spatial distributions. Causes of low estimated abundance and altered spatial distribution for KIMU in 2019 are unknown, and these data should be interpreted cautiously. Large annual fluctuations in abundance estimates of both species have quickly reversed transient changes, and future estimates will inform interpretation of 2019 results. Fluctuations likely reflect multiple factors, including: imprecision of estimates, potential bias in estimates resulting from infrequent but pervasive misidentification of these similar species, and true change in populations due to demographic factors such as survival, reproduction, and regional movements among alternative summering areas. Recent advances in sampling and analytic methods have highlighted challenges presented by misidentification, but also offer opportunity for enhancing monitoring methods to minimize misidentification and consequent degraded capacity to meet objectives of monitoring abundance and trend. Glacier Bay continues to house a significant fraction of the global population of each species. Line transect methods require accurate distance estimates to animals to accurately estimate abundance. Distance calibration, which develops proficiency of survey observers in distance estimation, was improved by new binocular rangefinders allowing quick target acquisition (both buoys and murrelets) at previously under-represented longer distances (125–200 m) that are commonly used during surveys. These capabilities increased realism of distance calibration and demonstrated that observers met performance objectives, even at longer distances.

Monitoring Kittlitz’s and marbled murrelets in Glacier Bay National Park and Preserve: 2019 annual report

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